Why is Blender Guru UV unwrapping?
The cube on the right is flattened/unfolded so a texture can be applied. On the left is what it looks like after it is unfolded. Where it is cut to unfold these are called seams. In the video he places the seams to unfold the object.



By creating a 2D view of the object you can add a texture in an external editor (for example) exactly where you want the texture to be. The 2D version is mapped to the 3D object ... UV Map